Over-modulation alarm device



oct. 1, 1940. o. E, Dow 2,216,232

OVER-MODULATION ALARM DEVICE Filed NOV. 50, 1937 I :IC

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BY Mw/L ATToRNEY Patented Oct. 1,A 1940 PATENT oFElcE OVER-MODULATION ALARM DEVICE Orville E. Dow, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation oi" Delaware Application November 3o, 1937, serial No. 171,212

6 Claims.

My present invention relates to over-voltage and over-modulation alarm devices.

An object of the present invention is to provide an over-modulation alarm which is .sub- 6 stantially independent of the variations in power supply line voltage. -Another object of my invention is to provide an over-modulation alarm which is self-adjusting for small variations in carrier signal input. Still another object of my l invention is to improve generally the accuracyand eiiiciency of over-modulation alarm devices.

Briefly, my present invention involves a pentode amplier for the alternating current cornponent of the rectified carrier. The plate current of a pentode tube is substantially independent of the plate voltage applied thereto within reasonable limits when the potential on the screens is maintained constant at the proper operating point. Thescreen grid voltage of the amplifier is maintained constant by means of a voltage regulator tube. Furthermore, in order to make the amplifier self-adjusting for varying carrier amplitudes the D. C. component 'of the rectied signal may be used to control the pentode plate current. lA battery isused to supply normal grid bias to the amplier when the signal level is normal.` Y

For a given percentage modulation of the signal, the alternating current component of the rectified signalwillincrease proportionately to the increase in the direct current component. Therefore, since an increase in the carrier amplitude. causes an increase inthe plate current of the amplifier any increase in the carrier amplitude will require a correspondingly larger increase in the alternating current component to actuate the alarm.

Referring now tothe drawing for amore complete description of my present invention. F18. 1

shows a circuit diagram ofione form of my in- 4 vention`whileY Fig. 2-show's a modication which provides for a lcompensation for varying carrier amplitudes.` In-Fig. i, il and 'I2 represent the input to the devicetowhich the rectiiied modulatedcarrier is supplied.` In the following explanationfthe diil'erent electrode' voltages referred to are 'taken with respect to the cathode of the tube. The alternating current component of hthe signal isapplied to the grid `of amplifier 2l through condenser I9.` Tube 2| is a pentode 'amplifier tube and one of the characteristics` of this type of tube is that theplate current is substantially vindependent of the plate voltage within reasonable limits. The plate voltage offtube 2i 5 is supplied from rectifier which is connected to (Cl. P19-171.5)

the alternating current supply line. The screen voltage for tube 2| is also supplied by the rectifier 26 through resistance 20. The glow-discharge tube 30 connected to the screen of tube 2| maintains the screen voltage substantially constant in. 5 spite of variations in the D. C. supply voltage.

The amplified alternating current component of the signal appears in potentiometer 23 and is applied through condenser 21 to the grid of glowdischarge tube 22. The plate voltagev for the 10 glow-discharge tube 22 is supplied by transformer 25. The grid bias of tube 22 is determined by the settings of potentiometers 24 and 23 and by the direct current through these potentiometers.

For a condition of zero signal input, i. e., with 15 no modulation on the carrier, the slider of potentiometer 23 is placed at point A. The slider of potentiometer 24 is then adjusted until tube 22 is juston the point of conducting. The A. C. line` voltage maybe considered normal at this time. 20 'Ihe vmaximum plate voltage En of tube 22 is the peak voltage appearing across the secondary of transformer 25. The direct current bias of this tube is the voltage between the slider and point D `of potentiometer 24 and is just suici'ent to pre- 25 vent the tube from conducting. There is no. A. C. voltagedon the grid. Now, if the A. C. line voltage increases or decreases 10 percent the plate voltage of tube 22 will increase or decrease 10 percent. Also, the bias will correspondingly in- 30 crease or decrease. 10 percent and the tube is still operating at the-point of break-down, that is, any amplitude of the alternating current component ofthe rectified carrier applied across the input lil, l2 will cause thevtube to conduct and actuate 35 relay 32 to operate the alarm.

y For a condition' of ilxed signal input other than zero, the potentiometer 24 isleft as adjusted for the zero signal condition. The slider of potentiometer 2l is moved toward point B until the 40 tube 22 justy fails to conduct. The maximum plate voltage E of 22 is, as before, the peak voltage across the transformer 25. The bias voltageimpressed 'on the grid is the voltage E1 between the sliderand-point D of potentiometer 24, plus 45 the voltage 'En between the point A and the sliderl ofpotentiometer `23 -due to the plate current of tube 2l. 'Ihe A. C. voltage on the grid ci tubev22 is the A. C. component of the `signal amplified by gtube 2l and appearing across the whole of poten- 50' tiometer 23. The peak valueW 0L-'this voltage Ew' is equal to the D. C. voltage En between point A and the slider of potentiometer 23. Therefore, the minimum instantaneous bias of tube22 is equai to the voltage Eibetween the slider and 55 point D .of potentiometer 24. From the condition of zero signal it has been shown that the voltage E1 and the plate voltage of tube 22 maintain the proper relation to each other, as the line voltage is varied, to keep the tube 22 at the proper operating point. Now, iithe peak alternating current voltage Ew across the potentiometer 23 and the D. C. voltage En between the point A and the slider potentiometer 23 remain equal, the device is independent of line,voltage. The A. C. voltage Ew will be independent of line voltage so far as the ampliilcation of tube 2| is independent of plate voltage. It has already been shown that the direct current in potentiometer 23 is independent oi line voltage within reasonable limits and, therefore, Ez is independent ofthe line voltage within the same limits. Actual measurements show that the accuracy is decreased by only 1 percent when the line voltage changes by plus or minus l5 percent. l

For the device of Fig. lv to operatev accurately as an over-modulation alarm the direct current component of the signal input'must be kept at a predetermined value. In the prior art, as far as I am aware, this is usually done manually. The voltage on the radio frequency ytransmission line of a short wave or ultra-short wave transmitter varies with the weather conditions. Hence,.the rectiiied input to themonitor will vary and will require the input to be readjusted to its proper value. Fig. 2 shows a modification of the input circuit of Fig. 1 that makes it self-adjusting for small variations in vcarrier signal input. The battery 29 is provided which balances out the normal direct current component of the signal and also supplies `normal bias to tube 2l.4 If the carrier arises above normal values the plate current of tube 2| increases correspondingly. 'Ihis increases the bias on tubeV 22. For the'same percentage modulation of the signal the increase in the alternating current component of the signal is proportional to the increase ofthe direct current component. Thus, the A. C. voltage across potentiometer 23 increases by the same amount that the bias on tube 22 was increased and the circuit remains in its predetermined condition of sensitivity within reasonable limits.

A decrease in the plate current of tube 2| decreases the voltagedrop through resistor 23.` The potential Ez therefore becomes smaller and the potential on the slider on resistor 23 thus becomes more positive since the total voltage drop fromA point-D to point B is substantial proportional to the conductivity of the tube`2l. It will thus be seen that as the platecurrent of tube -2|' decreases the value of E: decreases and the grid .oi' tube 22 is rendered less negative. The negative bias on the grid of tube 22 due to the current ilow through potentiometer 2l from source 26 is substantially fixed and is oi' itself insuiilcient to pre-l vent conduction/of tube 22; Therefore, when E2 decreases' suiliciently the gridof tube 22 is rendered suillciently less negative that the tube becomes conductive and the alarm is thus ener- In-the above description the tube 22 was sup- -plied with A. C. voltage on',its plate. The purpose'oi' this is to return control to the grid when the discharge is extinguished on the negative hali'of the cycle. The tube 2 2 may be supplied with plate voltage from rectifier 28 if it is desired to have Ithe alarmv continue to operate lmtil it is reset by opening the plate circuit. 'I'he operation of the device willnot be changed.

While I have indicated and described a parrent component from said Signal;

'to the particular organization. shown and described and modications may be made without departing from the scope and spirit, of the invention.

I claim:

1. An over-modulation alarm device comprising an inputlcircuit for a rectied modulated signal; means for separating the alternating cur amplifying means for said alternating current component, said means comprising a pentode tube having a plate and a screen grid, a constant source of potential connected to said screen grid, a source -oi potential for said plate, a resistance connected between said plate and said source of potential, a grid controlled gas discharge tube having a grid and a plate means i'or normally biasing said grid controlled gas discharge tube inoperative and means responsive to a decrease in the plate current of, said pentode below a predetermined value for removing said bias and an alarm circuit connected to the plate of said grid controlled gas discharge tube.

2. A device of the character described comprising an input circuit fora rectiiied modulated carrier having a direct current and an alternating current component, an amplier including a pentode tube having a control grid, a screen grid and a plate,`a source of potential for biasing said control grid and for opposing the normal direct current component oi said carrier, a grid controlled gas discharge tube having grid and plate circuits, an alarm in the plate circuit of said gas discharge tube, a source oi' potential connected to the plates of said gas dischargeand said pentode tubes, said source being subject to voltage fluctuations, means for applying a constant potential tothe screen of said pentode tube whereby the platecurrent oi said tube is substantially unaffected by voltage iluctuations in said source; means for biasing said gas discharge tube inoperative and means for connecting said gas discharge tube gridcircuit to the pentode tube so that a change in the direct current component of the rectified signal produces a proportional change in said bias .whereby the alarm in the plate circuit of said gas discharge tube is operated.

3. An over-modulation alarm device comprising an input circuit fora rectified modulated signal, means `for separating the alternating current component from said signall, amplifying means for said alternating current component said means including a pentode tube having a plate and a screen grid, a grid controlled gas discharge tube having grid and platel circuits,

' means for normally biasing said grid controlled gas discharge tube inoperative, an alarm in the plate circuit 'of said gas discharge tube. means for so connecting the grid circuit of said g rclis charge tube to the plate of said pentode tube that a decrease in the plate current of said pentode below a predetermined value removes said bias and actuates said alarm, a power supply line subject to voltage iluctuations, means for energizing the plate circuit of said gas Vdischarge tube from said line, means energized from said line vfor supplying a direct potential to the plate of said pentode and toV said biasing means and means' including a second gas discharge tube vfor supplying a constant voltage to the screen grid of said pentode,

4. An over-modulation alarm device comprising an input circuit for a rectified modulated signal, means for separating the alternating current component from said signal, amplifying means for said alternating current component, said' means including a pentode tube vhaving a plate and a screen grid, a grid controlled gas discharge tube having grid and plate circuits, means for normally biasing said grid controlled gas dischargey tube inoperative, an alarm in the plate circuit of said gas discharge tube, means :for so connecting the grid circuit of said gas discharge tube to the plate of said pentode tube that a decrease in the plate current of said pentode below a predetermined value removes said bias and actuates said'alarm, an alternating current power supply line subject to voltage uctuations, means for energizing the plate circuit oi said gas discharge tube from said line, a rectiier connected to said line for supplying a direct4 potential to lthe plate of said pentode and to said biasing means and means including .a second gas discharge tube for supplying a constant voltage to the screen grid of said pentode.

5. An over-modulation alarm device comprising an input circuit for a rectified modulated signal, means for separatingthe alternating current component from said signal, a pentode tube having a control grid connected to said means, a cathode, a screen grid and a plate; a grid controlled gas discharge tube having a grid and a plate, .a condenser connected from the plate of said pentode to the gridof said gas discharge tube, an alternating current power line subject to voltage fluctuations, means for energizing the plate of said gas discharge tube from said line, an alarm connected between said plate and said means, a 'rectifier having its input terminals connected to said line and a voltage divider connected to its output, a connection including .a series resistance from vthe plate of said pentode to an intermediate point on said voltage divider, a resistance connected from the grid of said gas discharge tube to a tap on said plate resistance, a .connection from the cathode of said gas discharge tube to a point on said voltage divider more positive than said intermediate point whereby said gas discharge tube is normally'` biased inoperative, a second gas discharge tube and a resistance connected in series across said voltage divider, and a connection from thescreen grid of said pentode tube to the junction between said last resistance and said second gas discharge tube whereby a substantially constant potential is applied to said screen grid,

6. An over-modulation alarmr device comprising an input circuit for a rectiiied modulated signal, means for separating the alternating current component from said signal, a pentode tube having a control grid connected to said means, a cathode, a screen grid and a plate; a grid controlled gas discharge tubev having a grid and a plate, a condenser connected from the plate of said pentode to the grid of said gas discharge tube, an alternating current power line subject to voltage uctuations, means for renergizing the plate of said gas discharge tube from said line. an alarm connected between said plate and.said mea-ns, a rectifier having its input terminals connected to said line and a voltage divider connected to its output, a connection from the cathode of said pentode to the negative end of said voltage divider, the positive end of said potential divider being connected to said last mentioned means, a connection including a series resistance from the plate of said pentode to an intermediate point on said voltage divider, a resistance connected fromthe grid of said gas discharge tube to a tap on said plate resistance, a connection from the cathode of said gas discharge tubel to a point on said voltage divider more positive than said intermediate point whereby said 'gas discharge tube is normally biased inoperative, a

secondgas discharge tube and a resistance connected in series across said voltage divider, and

, a connection from the screen grid of said pentode tube to the junction between said last resistance and said second gas discharge tube whereby a substantially constant potential is applied to said screen grid. v

ORVILLE E. DOW. 

